Resource use by juvenile brown trout andAlpine bullhead: influence of interspecific versus intraspecific competition

AbstractStream‐dwelling salmonids and bullheads occupy similar resource niches in northern rivers. It is therefore tempting to assume that they might be involved in a competitive interaction, with potential implications for the habitat use and growth of brown trout (Salmo trutta). We conducted artificial‐stream experiments to test whether a putative competitor,Alpine bullhead (Cottus poecilopus), had an effect on the habitat use of under‐yearling (0+) and yearling (1+) trout. We hypothesised that (i) 1+ trout would be competitively superior to 0+ trout, forcing the younger fish to suboptimal habitats, and that (ii) bullhead might affect the habitat use and prey selection of 0+ trout but less so that of 1+ trout. Against our predictions, no effect of bullhead was found on the habitat use of either age class of brown trout. Instead, yearling trout seemed to force bullheads to suboptimal microhabitats with high current velocities. Presence of yearlings also decreased the growth of under‐yearling fish and caused a shift in their diet composition. These findings suggest that competitive interactions may not limit the coexistence of brown trout and bullheads in boreal rivers. Intraspecific interactions between trout age classes may be more important, with potentially detrimental effects on the growth and overwintering success of 0+ trout.